empty strings: tabStaff insists on stringnumbers and looses notes

[s . runkel]

Hi, 

Just upgraded from 2.12 to 2.14.2.

Problem:
Compiling the example below results in a waring in the log file:
Warnung: Keine Saite fuer Tonhoehe #<Pitch c > (Bund (5) angegeben)

and the c note is lost.
However, that note was shown as the empty 4th string in 2.12.


\score {    
  \new TabStaff {
    % cister-tuning defined in string-tunings-init.ly:
    % (cister-tuning . ,#{<c g d' a'>#})
    \set Staff.stringTunings  = #cister-tuning 
    <c g'> 1 <c\4 g'> 1
  }
}

results in:
---------- 
---5--5---
----------
------0---

But should read as in 2.12:
---------- 
---5--5---
----------
---0--0---